RFC: Nest.js migration

[apalchys]

There is a strong willing to migrate back-end to Nest.js from Koa.

Reasons:

• well-stablished patterns
• good community support
• active development
• a lot of built-in functionality
• swagger out-of-box

Additionally, we need to address the following issues:

• Provide an easy way to build separate applications on top of RS School REST API (RSS Teams app is a good example)
• Support GitHub username rename
• Multi-device (session is stored inside the cookie)

[apalchys]

@AlreadyBored @valerydluski @dzmitry-varabei @Pulya10c @mikhama Looking for your feedback and comments